The semiconductor industry is continuously trying to increase the density of devices by means of recent state-of-the-art semiconductor device processing techniques. Of all the techniques, deposition and planarization processes of dielectric films including interlayer dielectric films have lately attracted considerable attention and are expected to play an important role in the semiconductor industry in the future to come. Quality semiconductor devices much depend upon the planarization process. Generally, semiconductor devices have many layers. As semiconductor device dimensions are reduced to submicron regions, upper layer formation is strongly effected by the undulation of a lower layer. The degree of the planarization increasingly becomes important and the thermal burden, which is known as the thermal budget in the art, increases in order to planarize a dielectric film. But thermal processes influence on the transistor and deteriorate the device characteristics. On the other hand, such a thermal budget of processing must be reduced to suppress the short-channel effects in the transistor.
With a view to reducing the budget mentioned above, chemical vapor deposition (CVD) process is commonly used to deposit a film of phosphorus-doped silicon dioxide (i.e., phosphosilicon-glass, PSG) or a film of boron-and-phosphorus-doped silicon dioxide (i.e., borophosphosilicate glass, BPSG). A film of PSG flows at about 950.degree. C. A BPSG film as an interlayer dielectric film, when subjected to thermal processing at a low temperature, say about 900.degree. C., flows for its planarization.
If the deposition of dielectric films, formed by an impurity-doped silicate glass such as PSG, BPG, BPSG and AsSG, is carried out by means of a conventional CVD process, their respective hydrides, chlorides, and organic compounds such as alkoxide are introduced (see Table). Silicate glass finds its use in semiconductor devices recently.
TABLE ______________________________________ Source Compounds ______________________________________ 1. Phosphorus PH.sub.3, POCl.sub.3, PCl.sub.3, PO(OCH.sub.3).sub.3, PO(OC.sub.2 H.sub.5).sub.3, P(OCH.sub.3).sub.3, P(OC.sub.2 H.sub.5).sub.3 PO(CH.sub.3).sub.3, P(CH.sub.3).sub.3 2. Boron B.sub.2 H.sub.6, BCl.sub.3, B(OCH.sub.3).sub.3, B(OC.sub.2 H.sub.5).sub.3, B(CH.sub.3).sub.3, B(C.sub.2 H.sub.5).sub.3 3. Arsenic AsH.sub.3, As(OCH.sub.3).sub.3, As(CH.sub.3).sub.3 ______________________________________
An example is given to describe a conventional semiconductor device and its fabrication process, with the help of the appended drawings. FIG. 8 locally Illustrates, in cross section, a conventional semiconductor device. A semiconductor substrate is indicated by reference numeral 1, a switching transistor by reference numeral 2, a silicon dioxide film by reference numeral 3, a BPSG film, which is deposited onto the silicon dioxide film 3, by reference numeral 6, and an aluminum wiring by reference numeral 8, a polycrystalline silicon as a drain or a source electrode by reference 11, and an n-type impurity by reference numeral 12.
The BPSG 6 film is deposited, through a CVD process, onto the silicon substrate 1 by making use of a source gas formed by doping TEOS (tetra-ethyl-ortho-silicate, Si(OC.sub.2 H.sub.5).sub.4), SiH.sub.4 (monosilane gas), and the like for forming a silicon dioxide film (i.e., a matrix), with phosphorus and boron sources. When subjected to thermal processing at a temperature of about 900.degree. C., the BPSG film 6 is planarized with a smooth flow shape.
FIG. 9 is a diagram outlining a conventional deposition of a film of BPSG. As shown in the figure, SiH.sub.4, B.sub.2 H.sub.6, PH.sub.3, and O.sub.2 are used as a source gas while N.sub.2 is used as a carrier gas. These gases are introduced into a reaction chamber 10 to deposit the BPSG film 6 onto the silicon substrate 1. Here, an atmospheric pressure CVD system is employed. Then the silicon substrate 1 is heated to somewhere between 350.degree. and 450.degree. C. The deposited BPSG film 6 is placed inside an electric furnace, being subjected to thermal processing for about 30 minutes at about 900.degree. C. in an atmosphere of nitrogen. The BPSG film 6 flows upon heating. At this point in time, the flow angle .theta. of the BPSG film 6 must be about 30 or less degrees for the planarization thereof to be done.
FIG. 10 Illustrates an internal structure of the post-flow BPSG film 6 formed by the above processing. The BPSG film 6 is here shown in plane for the sake of simplification but actually has a 3-D network structure formed by the B.sub.2 O.sub.3 -P.sub.2 O.sub.5 -SiO.sub.2 bond. More specifically, the atom of silicon serves as a main constituent, whereas the atom of oxygen serves as a sub-constituent which forms a bond at the tetrahedron position of the silicon atom. Both the main and sub-constituents bond together in a network fashion to form a matrix of silicate glass. Such a network matrix has a structure in which phosphorus and boron atoms replaces a silicon atom.
The increased density of semiconductor devices involves a risk of the short-channel effects. In order to suppress the unwanted short-channel effects, a film of BPSG must undergoes thermal processing at temperatures lower than 900.degree. C. If, however, the thermal processing is carried out at below 900.degree. C., the degree of planarization will get worse because the BPSG film does not flow sufficiently. This is explained by the fact that the flow angle .theta. of the BPSG film varies with the thermal processing temperature. In other words, the flow angle increases as the thermal processing temperature decreases.
There are two processes known in the art in order to achieve lower-temperature thermal processing of dielectric films such as a BPSG film while still keeping the flow angle as small as about 30 degrees.
(Method I)
In accordance with this method, thermal processing is carried out in an atmosphere of high-pressurized hot air or in an atmosphere of water vapor, in order to reduce the flow angle .theta..
(Method II)
In this method, the dose of impurities such as phosphorus, boron and arsenic into a dielectric film is increased to reduce the flow angle .theta..
The Method I, however, requires the provision of an anti-oxidizing dielectric film formed by, for example, silicon nitride underneath a film of BPSG to protect a transistor. Those which require many fabrication steps such as DR are unable to take advantage of this method because it is impossible to form a film of silicon nitride. This method is too much limited to be practical.
The Method II, on the other hand, may suffer a risk that precipitated substances are produced, although the degree of planarization can be improved by increasing the concentration of impurities. This means that less impurity dose (phosphorus, boron and the like elements) is preferable. For the case of a BPSG film, it is likely that the atom of boron condense to form a core, the core reacting with a phosphooxide gas given off from the film resulting in the crystal growth of BPO.sub.4. This results in the formation of a "bump". The denser the concentration of impurities becomes, the more foreign substances are produced. This arises a problem in a DRAM that its upper-layer bit line or aluminum wiring is likely to be disconnected, which results in poor-quality semiconductor devices.
It has been widely held that to lower the temperature of thermal processing conflicts with the planarization, and that the margin of material has been reached.